Showing papers on "Pentacene published in 2008"

Self-Organization of Ink-jet-Printed Triisopropylsilylethynyl Pentacene via Evaporation-Induced Flows in a Drying Droplet†

Abstract: We have demonstrated the influence of evaporation-induced flow in a single droplet on the crystalline microstructure and film morphology of an ink-jet-printed organic semiconductor, 6,13-bis((triisopropylsilylethynyl) pentacene (TIPS_PEN), by varying the composition of the solvent mixture. The ringlike deposits induced by outward convective flow in the droplets have a randomly oriented crystalline structure. The addition of dichlorobenzene as an evaporation control agent results in a homogeneous film morphology due to slow evaporation, but the molecular orientation of the film is undesirable in that it is similar to that of the ring-deposited films. However, self-aligned TIPS_PEN crystals with highly ordered crystalline structures were successfully produced when dodecane was added. Dodecane has a high boiling point and a low surface tension, and its addition to the solvent results in a recirculation flow in the droplets that is induced by a Marangoni flow (surface-tension-driven flow), which arises during the drying processes in the direction opposite to the convective flow. The field-effect transistors fabricated with these self-aligned crystals via ink-jet printing exhibit significantly improved performance with an average effective field-effect mobility of 0.12 cm2 V–1 s–1. These results demonstrate that with the choice of appropriate solvent ink-jet printing is an excellent method for the production of organic semiconductor films with uniform morphology and desired molecular orientation for the direct-write fabrication of high-performance organic electronics.

Structure and Properties of Small Molecule−Polymer Blend Semiconductors for Organic Thin Film Transistors

Abstract: A comprehensive structural and electrical characterization of solution-processed blend films of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) semiconductor and poly(α-methylstyrene) (PαMS) insulator was performed to understand and optimize the blend semiconductor films, which are very attractive as the active layer in solution-processed organic thin-film transistors (OTFTs). Our study, based on careful measurements of specular neutron reflectivity and grazing-incidence X-ray diffraction, showed that the blends with a low molecular-mass PαMS exhibited a strong segregation of TIPS-pentacene only at the air interface, but surprisingly the blends with a high molecular-mass PαMS showed a strong segregation of TIPS-pentacene at both air and bottom substrate interfaces with high crystallinity and desired orientation. This finding led to the preparation of a TIPS-pentacene/PαMS blend active layer with superior performance characteristics (field-effect mobility, on/off ratio, and threshold voltage)...

Tuning of Ag work functions by self-assembled monolayers of aromatic thiols for an efficient hole injection for solution processed triisopropylsilylethynyl pentacene organic thin film transistors

Abstract: We demonstrate tuning of hole injection barriers in bottom contact triisopropylsilylethynyl pentacene (TIPS-pentacene) organic thin film transistors (OTFTs) by forming the self-assembled monolayers (SAMs) of thiophenol, 4-fluorothiophenol, or pentafluorothiophenol on the pristine Ag electrode. The work functions of SAM-treated Ag electrodes are measured by Kelvin probe method. The TIPS-pentacene OTFT devices were fabricated by a drop-cast method with a micropipette like an inkjet printing. The OTFTs with pentafluorothiophenol-Ag electrodes as source and drain exhibit carrier mobility of 0.17cm2∕Vs and on/off current ratio of 105 because of almost no hole injection barrier to TIPS pentacenes. The SAM-treated Ag electrodes are robust over repeated electrical scans of 100cycles.

Pentacene-based organic field-effect transistors

Abstract: Organic field-effect transistors (FETs) have attracted considerable attention because of their potential for realizing large-area, mechanically flexible, lightweight and low-cost devices. Pentacene, which is a promising material for organic FETs, has been intensely studied. This article reviews the basic properties of pentacene films and crystals, and the characteristics of pentacene FETs fabricated under various conditions, including our recent achievement of low-voltage operating high-mobility FETs. The basic properties include the crystal polymorph, the band structure and the effective mass. These data have been used for discussion of carrier transport and mobility in pentacene films. The characteristics of pentacene FETs generally depend on the conditions of the pentacene film and the gate-dielectric surface. The dependences are summarized in the article. In addition, liquid-crystal displays and organic light-emitting device arrays using pentacene FETs are reviewed as applications of organic FETs, and complementary metal–oxide–semiconductor circuits using our low-voltage operating FETs are also shown.

Effect of the phase states of self-assembled monolayers on pentacene growth and thin-film transistor characteristics.

Abstract: To investigate the effects of the phase state (ordered or disordered) of self-assembled monolayers (SAMs) on the growth mode of pentacene films and the performance of organic thin-film transistors (OTFTs), we deposited pentacene molecules on SAMs of octadecyltrichlorosilane (ODTS) with different alkyl-chain orientations at various substrate temperatures (30, 60, and 90 degrees C). We found that the SAM phase state played an important role in both cases. Pentacene films grown on relatively highly ordered SAMs were found to have a higher crystallinity and a better interconnectivity between the pentacene domains, which directly serves to enhance the field-effect mobility, than those grown on disordered SAMs. Furthermore, the differences in crystallinity and field-effect mobility between pentacene films grown on ordered and disordered substrates increased with increasing substrate temperature. These results can be possibly explained by (1) a quasi-epitaxy growth of the pentacene film on the ordered ODTS monolayer and (2) the temperature-dependent alkyl chain mobility of the ODTS monolayers.

Origin of the open-circuit voltage in multilayer heterojunction organic solar cells

Abstract: From temperature dependent studies of pentacene/C60 solar cells in the dark, the reverse saturation current is found to be thermally activated with a barrier height that corresponds to the difference in energy between the highest occupied molecular orbital of the donor and the lowest unoccupied molecular orbital of the acceptor corrected for vacuum level misalignments and the presence of charge-transfer states. From the reverse saturation current in the dark and the short-circuit current under illumination, the open-circuit voltage can be predicted. Examination of several donor materials supports the relationship between reverse saturation current, this barrier height, and open-circuit voltage.

Coulomb barrier for charge separation at an organic semiconductor interface.

Abstract: Charge transfer (CT) excitons across donor-acceptor interfaces are believed to be barriers to charge separation in organic solar cells, but little is known about their physical characteristics. Here, we probe CT excitons on a crystalline pentacene surface using time-resolved two-photon photoemission spectroscopy. CT excitons of $1s$, $2s$, and $3s$ characters are bound by Coulomb energies of 0.43, 0.21, 0.12 eV, respectively, in agreement with quantum mechanical modeling. The large binding energy of the $1s$ CT exciton excludes its participation in photovoltaics. Efficient charge separation in organic heterojunction solar cells must involve a series of hot CT excitons.

Charge Trapping in Intergrain Regions of Pentacene Thin Film Transistors

Abstract: A scanning Kelvin probe microscopy (SKPM) study of the surface potential of vacuum sublimed pentacene transistors under bias stress and its correlation with the film morphology is presented. While for thicker films there are some trapping centers inhomogeneously distributed over the film, as previously reported by other authors, by decreasing the film thickness the effect of thin intergrain regions (IGRs) becomes clear and a very good correlation between the topography and the potential data is observed. It is shown that in the thick pentacene grains the potential is homogeneous and independent of the gate bias applied with negligible charge trapping, while in the thin IGRs the potential varies with the applied gate bias, indicating that only an incomplete accumulation layer can be formed. Clear evidence for preferential charge trapping in the thin IGRs is obtained.

Tuning the Ionization Energy of Organic Semiconductor Films: The Role of Intramolecular Polar Bonds

Abstract: For the prototypical conjugated organic molecules pentacene and perfluoropentacene, we demonstrate that the surface termination of ordered organic thin films with intramolecular polar bonds (e.g., -H versus -F) can be used to tune the ionization energy. The collective electrostatics of these oriented bonds also explains the pronounced orientation dependence of the ionization energy. Furthermore, mixing of differently terminated molecules on a molecular length scale allows continuously tuning the ionization energy of thin organic films between the limiting values of the two pure materials. Our study shows that surface engineering of organic semiconductors via adjusting the polarity of intramolecular bonds represents a generally viable alternative to the surface modification of substrates to control the energetics at organic/(in)organic interfaces.

Moisture induced electron traps and hysteresis in pentacene-based organic thin-film transistors

Abstract: Previous work revealed that electron traps in pentacene formed in air cause hysteresis in thin-film transistor characteristics Here, we experimentally showed that water vapor, rather than oxygen, is responsible for these hysteresis-causing trap states Photogenerated and injected electrons are trapped at the pentacene-dielectric interface under positive gate bias, and induce extra holes, resulting in the observed extra drain current The electron detrapping causes the decay of the extra hole population with time under negative gate bias and, therefore, that of the drain current

Anisotropic mobility in large grain size solution processed organic semiconductor thin films

Abstract: The hollow pen method for writing thin films of materials from solution is utilized to deposit films of 6,13-bis(tri-isopropylsilylethynyl) pentacene (TIPS pentacene) onto SiO2 surfaces with pre-patterned source/drain gold contacts. We demonstrate that large domains are obtained for TIPS pentacene films deposited from 0.5–4.0wt% solutions with toluene. Crystalline grains with (001) orientation are observed to grow with sizes that can exceed 1mm along the writing direction. A preferred azimuthal orientation is also selected by the process, resulting in anisotropic field effect transistor mobility in the films.

Tunable Crystal Nanostructures of Pentacene Thin Films on Gate Dielectrics Possessing Surface-Order Control†

Abstract: To enhance the electrical performance of pentacene-based field-effect transistors (FETs) by tuning the surface-induced ordering of pentacene crystals, we controlled the physical interactions at the semiconductor/gate dielectric (SiO2) interface by inserting a hydrophobic self-assembled monolayer (SAM, CH3-terminal) of organoalkyl-silanes with an alkyl chain length of C8, C12, C16, or C18, as a complementary interlayer. We found that, depending on the physical structure of the dielectric surfaces, which was found to depend on the alkyl chain length of the SAM (ordered for C18 and disordered for C8), the pentacene nano-layers in contact with the SAM could adopt two competing crystalline phases—a “thin-film phase” and “bulk phase” – which affected the π-conjugated nanostructures in the ultrathin and subsequently thick films. The field-effect mobilities of the FET devices varied by more than a factor of 3 depending on the alkyl chain length of the SAM, reaching values as high as 0.6 cm2 V−1 s−1 for the disordered SAM-treated SiO2 gate-dielectric. This remarkable change in device performance can be explained by the production of well π-conjugated and large crystal grains in the pentacene nanolayers formed on a disordered SAM surface. The enhanced electrical properties observed for systems with disordered SAMs can be attributed to the surfaces of these SAMs having fewer nucleation sites and a higher lateral diffusion rate of the first seeding pentacene molecules on the dielectric surfaces, due to the disordered and more mobile surface state of the short alkyl SAM.

Hysteresis mechanisms of pentacene thin-film transistors with polymer/oxide bilayer gate dielectrics

Abstract: We have studied the electrical stability of organic poly-4-vinyl phenol (PVP)/inorganic oxide bilayer gate dielectrics for low-voltage pentacene thin-film transistors (TFTs). Curing conditions of spin-cast PVP influence on the drain current-gate bias hysteresis behavior; long term curing reduces the magnitude of the hysteresis, which can also be reduced by decreasing the PVP thickness. The electron charge injection from gate electrode plays as another cause of the electrical hysteresis. These instabilities are categorized into the following three: channel/dielectric interface-induced, slow polarization-induced, and gate charge injection-induced hystereses. By examining the hysteresis behavior of pentacene TFTs with five different combinations of bilayer dielectric, we clarified the instability mechanisms responsible for the electrical hysteresis.

Reducing the contact resistance of bottom-contact pentacene thin-film transistors by employing a MoOx carrier injection layer

Abstract: We report on the reduced contact resistance in bottom-contact (BC) pentacene thin-film transistors (TFTs) with a molybdenum oxide (MoOx) carrier injection layer MoOx layers were placed between the gate insulator and the source-drain (S-D) electrodes instead of the conventional adhesive layer such as Cr or Ti The performance of the BC pentacene-TFT with the MoOx injection layer was significantly improved at low operating voltages The contact resistance of the MoOx∕Au S-D electrodes, estimated using the gated-transmission line method, was nearly two orders of magnitude smaller than that of conventional Cr∕Au electrodes at the gate voltage of −10V The highest performance was obtained with a MoOx injection layer a few nanometers thick, which was comparable to the effective channel thickness of the pentacene-TFT on the gate insulator This result indicated the importance of the direct connection between the MoOx injection layer and the effective channel to reduce the contact resistance

Solvent-dependent electrical characteristics and stability of organic thin-film transistors with drop cast bis(triisopropylsilylethynyl) pentacene

Abstract: The solvent from which the active layer is drop cast dramatically influences the electrical characteristics and electrical stability of thin-film transistors comprising bis(triisopropylsilylethynyl) pentacene. Casting from high boiling solvents allows slower crystallization; devices cast from toluene and chlorobenzene thus exhibit mobilities >0.1 cm2/V s and on/off ratios of ∼106. More importantly, the solvent choice influences the device stability. Devices from toluene exhibit stable characteristics, whereas devices from chlorobenzene show hystereses on cycling, with dramatic threshold voltage shifts toward positive voltages. The instability in chlorobenzene devices is attributed to the migration of water and solvent impurities to the charge transport interface on repetitive testing.

Organic field effect transistors from ambient solution processed low molar mass semiconductor–insulator blends

Abstract: The morphology and organic field effect transistor (OFET) properties of two component blends of semi-crystalline 6,13-bis(triisopropylsilylethinyl)pentacene (TIPS-pentacene) with selected amorphous and semi-crystalline side chain aromatic low permittivity insulating binders deposited at room temperature under vacuum from a good solvent are reported. When blended with an amorphous binder there is evidence from X-ray photoelectron spectroscopy (XPS) of a strong interaction between TIPS-pentacene and the binder in the solidified film giving rise to twisted TIPS-pentacene crystals containing dislocations. Due to this strong interaction we see no evidence of segregation of TIPS-pentacene towards the active interface and hence we observe a rapid fall off in saturated hole mobility at an active concentration less than 50 wt%. When blended with a crystalline binder there is no evidence from XPS of any interaction between TIPS-pentacene and the binder in the solidified film. We propose that when a semi-crystalline binder is used, which crystallizes more slowly from solution than TIPS-pentacene, we observe stratification of the active material to both interfaces and as a result retention of saturated hole mobility even down to 10 wt%. The potential applications of the approach are in the formulation of low-cost organic semiconductors whose solution and solid state properties can be fine-tuned by careful binder selection.

Oxygen-related traps in pentacene thin films : Energetic position and implications for transistor performance

Abstract: We studied the influence of oxygen on the electronic trap states in a pentacene thin film. This was done by carrying out gated four-terminal measurements on thin-film transistors as a function of temperature and without ever exposing the samples to ambient air. Photo-oxidation of pentacene is shown to lead to a peak of trap states centered at 0.28 eV from the mobility edge, with trap densities of the order of ${10}^{18}\text{ }{\text{cm}}^{\ensuremath{-}3}$. As the gate voltage is ramped up, these trap states are occupied at first and cause a reduction in the number of free carriers at a given gate voltage. Moreover, the exposure to oxygen reduces the mobility of the charge carriers above the mobility edge. We correlate the change of these transport parameters with the change of the essential device parameters, i.e., subthreshold performance and effective field-effect mobility. This study supports the assumption of a mobility edge for charge transport and contributes to a detailed understanding of an important degradation mechanism of organic field-effect transistors. Deep traps in an organic field-effect transistor reduce the effective field-effect mobility by reducing the number of free carriers and their mobility above the mobility edge.

Grain-boundary-limited charge transport in solution-processed 6,13 bis(tri-isopropylsilylethynyl) pentacene thin film transistors

Abstract: Grain boundaries play an important role in determining the electrical, mechanical, and optical properties of polycrystalline thin films. A side-disubstituted counterpart of pentacene, 6,13 bis(tri-isopropylsilylethynyl) (TIPS) pentacene, has lateral π-π packing and reasonably high solubility in a number of organic solvents. In this paper, the effects of grain boundaries on the effective hole mobility, on/off ratio, threshold voltage, and hysteresis of transistor transfer characteristics were investigated in solution-processed TIPS pentacene thin film transistors with both experiments and simulations. The effects of solvent type, concentration, substrate temperature, and evaporation rate were investigated by optical, electron, and atomic force microscopies. An apparatus for controlled solution casting was designed, fabricated, and used to make TIPS pentacene thin film transistors with more precisely controlled variations in microstructure and defect densities. First, hysteresis in the electrical characteri...

Gold nanoparticle-pentacene memory-transistors

Abstract: We demonstrate an organic memory-transistor device based on a pentacene-gold nanoparticles active layer. Gold (Au) nanoparticles are immobilized on the gate dielectric (silicon dioxide) of a pentacene transistor by an amino-terminated self-assembled monolayer. Under the application of writing and erasing pulses on the gate, large threshold voltage shift (22V) and on/off drain current ratio of ∼3×104 are obtained. The hole field-effect mobility of the transistor is similar in the on and off states (less than a factor of 2). Charge retention times up to 4500s are observed. The memory effect is mainly attributed to the Au nanoparticles.

Mobile Ionic Impurities in Poly(vinyl alcohol) Gate Dielectric: Possible Source of the Hysteresis in Organic Field-Effect Transistors†

Abstract: Threshold voltage shifts in organic field effect transistors (OFETs) have been reported frequently. Cyclic sweeps of the gate voltage in OFETs reveal a hysteresis in the transfer characteristics (drain-source current versus gate-source voltage) thereby unfolding an electrical instability of the transistor element. On the one hand bistable transistors with a nonvolatile hysteresis in the transfer characteristics may be used in organic memory elements, whereas on the other hand hysteresis free transistors are desired in integrated organic circuits. Therefore understanding the causes of these electrical bior instabilities in organic field-effect devices is of primary interest. OFETs using charged electrets or ferroelectric-like gate dielectrics (e.g., see [7,8,12,14]) show a hysteresis which is normally attributed to the intrinsic properties of these materials. In polymer dielectrics without an intrinsic hysteresis such as poly(vinyl alcohol) (PVA), gate voltage induced hysteresis are often interpreted in terms of electrostatic screening of trapped charge carriers released under the influence of the electric field between the gate and the source/drain electrodes. Experimental results so far revealed a fairly complex picture of this “bias-stress” effect where the detailed mechanisms involved are not yet fully understood. Several publications report evidences of a charge trapping process in the organic semiconductor close to the semiconductor/dielectric interface. Lindner et al. supposed that the origin of the hysteresis in organic devices is a combination of slow transport (polarons or mobile ions) with a reaction other than trap recharging, e.g., a direct polaron-bipolaron reaction or a complex formation reaction of polarons / bipolarons with counterions. Bias-stress experiments with pentacene on various inorganic dielectrics indicate a reversible structural change of the semiconductor. A substantial number of experiments demonstrate an influence of the gate dielectric material on the formation of hysteresis effects. Transistors with a polymer gate dielectric are more likely to show a pronounced hysteresis than transistors with an inorganic gate dielectric. It has been reported that hydroxyl groups in the form of silanols at the SiO2-dielectric/semiconductor interface can work as electron traps which can be eliminated by a thin alkane interlayer. In many organic dielectrics hydroxyl groups are present in large numbers, either as integral part of the chemical structure and/or as impurities remaining from the process of synthesis. Such hydroxyl groups therefore were suspected of being responsible for the hysteresis effect in many OFET configurations. Lee et al. conclude that the increase of hydroxyl groups in polymer dielectrics equally increases electron trapping sites which in turn cause a large hysteresis in OFETs, but as they also discussed, the drain current as well as the gate leakage current increased with increasing hydroxyl density, which may be in contradiction with the proposed idea of immobilized carriers. Polymer dielectrics are often hosts for mobile ions. It has been demonstrated for example that mobile Na ions may diffuse from an underlying substrate into organic semiconductors, like pentacene or poly(3-hexylthiophene) under the influence of an applied voltage. Thereby they cause an increase of the current through the semiconductor and, additionally, a current-voltage hysteresis. The finding that mobile ions in semiconductors alter their electrical performance is not new. p-n junction devices, called flexodes, with a variable current-voltage (I–V) characteristics resulting from a reversible drift of Li ions were suggested in 1963. Two years later, small traces of mobile alkali ions in SiO2 gate dielectrics were reported to cause significant problems in semiconductor devices. In fact, the practical application of MOSFETs was delayed in the early 1960s because of severe gate bias instability problems caused by mobile ionic oxide charges like Na, C O M M U N IC A TI O N

Structural and electronic properties of pentacene-fullerene heterojunctions

Abstract: In this study the performance differences of layered and bulk-heterojunction based organic solar cells composed of the prototypical p- and n-type organic semiconductors pentacene (PEN) and fullerene (C60) are correlated with the physical properties of the heterostructures. The electronic structure of layered and codeposited thin PEN and C60 films on the conducting polymer substrate poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was investigated with ultraviolet photoelectron spectroscopy. Layered structures of C60 on PEN precovered PEDOT:PSS exhibited an offset of the highest occupied molecular orbital (HOMO) levels of 1.45 eV. In contrast, codeposited films of PEN and C60 showed a reduced HOMO-level offset of 0.85 eV, which increased to 1.45 eV by precoverage of the substrate with a thin PEN layer. In this case, the PEN-HOMO level was Fermi-level pinned at 0.35 eV binding energy and charge transfer between PEN and PEDOT:PSS decreased the vacuum level by 0.75 eV. In addition, the morpholo...

Solution-deposited zinc oxide and zinc oxide/pentacene bilayer transistors: high mobility n-channel, ambipolar and nonvolatile devices

Abstract: A solution processed n-channel zinc oxide (ZnO) field effect transistor (FET) was fabricated by simple dip coating and subsequent heat treatment of a zinc acetate film. The field effect mobility of electrons depends on ZnO grain size, controlled by changing the number of coatings and zinc acetate solution concentration. The highest electron mobility achieved by this method is 7.2 cm2 V−1 s−1 with On/Off ratio of 70. This electron mobility is higher than for the most recently reported solution processed ZnO transistor. We also fabricated bilayer transistors where the first layer is ZnO, and the second layer is pentacene, a p-channel organic which is deposited by thermal evaporation. By changing the ZnO grain size (or thickness) this type of bilayer transistor shows p-channel, ambipolar and n-channel behavior. For the ambipolar transistor, well balanced electron and hole mobilities are 7.6 × 10−3 and 6.3 × 10−3 cm2 V−1 s−1 respectively. When the ZnO layer is very thin, the transistor shows p-channel behavior with very high reversible hysteresis. The nonvolatile tuning function of this transistor was investigated.

Pentacene‐Zinc Oxide Vertical Diode with Compatible Grains and 15‐MHz Rectification

Abstract: Vertical p-n junction organic-inorganic hybrid diodes are fabricated and characterized. The current density and rectification speed in the open atmosphere are higher than previously reported organic vertical diodes with relatively lower operating voltage, and the stability is encouraging.

Correlation between grain size and device parameters in pentacene thin film transistors

Abstract: We develop a general approach to precisely extract the device parameters in top-contact pentacene thin film transistors. The charge trap sites are clarified by analyzing the grain size dependence of the device parameters. The channel mobility and threshold voltage are limited by the charge traps in the channel region, most of which are located not at the grain boundaries but at the organic/insulating-layer interface. The contact resistance decreases by increasing the grain size and is controlled by the charge traps in the contact region, which are suggested to be concentrated at the grain boundaries and at the metal/organic interface.

Bias stress instability in pentacene thin film transistors: Contact resistance change and channel threshold voltage shift

Abstract: Bias stress instability in top-contact pentacene thin film transistors was observed to be correlated not only to the channel but also to the metal/organic contact. The drain current decay under bias stress results from the combination of the contact resistance change and the threshold voltage shift in the channel. The contact resistance change is contact-metal dependent, though the corresponding channel threshold voltage shifts are similar. The results suggest that the time-dependent charge trapping into the deep trap states in both the contact and channel regions is responsible for the bias stress effect in organic thin film transistors.

Direct inkjet printing of silver electrodes on organic semiconductors for thin-film transistors with top contact geometry

Abstract: We have fabricated high-performance organic thin-film transistors with top contact geometry using silver source and drain electrodes that are directly patterned on pentacene film by inkjet printing. Ink droplets containing Ag nanoparticles were ejected from inkjet nozzles. By controlling wave forms applied to piezoelectric actuators in these nozzles, the volume of the droplets can be changed from 17 to 1.4 pl. When the volume of the droplets is reduced, dc characteristics of manufactured transistors are improved significantly. The transistors manufactured with droplets of 1.4 pl exhibit a mobility of 0.3 cm2/Vs and an on-off current ratio that exceeding 106.